Heating method



United States Patent Office 3,106,594 Patented Oct. 8, 1963 3,106,594HEATING METHOD .lack M. Beasley, Grand Prairie, and Herbert Greenewald,Jr., Dallas, Tex., assignors, by mesne assignments, toLing-Temco-Vought, Inc., Dallas, Tex., a corporation of Delaware FiledAug. 11, 1961, Ser. No. 130,830 9 Claims. (Cl. 13-1) This inventionrelates to methods for electric heating, and more particularly to a-method for heating an enclosure with a plasma.

High-temperature electric furnaces have previously fallen into fourprincipal groups when classified according to the metho-d of heatingemployed. These fou-r groups have included arc furnaces; furnacesemploying a solid resistance element; furnaces employing a liquidresistance element; and electron-beam furnaces. All these devices -havehad ce-rtain disadvantages and limitations, and each `feature making anyone of them attractive for a given, particular utilization is generallyoffset lby attendant and previously unavoidable disadvantages.

Thus, while the current flow to a furnace employing a solid or liquidresistance element is easily stopped and restarted as required toprovide the alternate periods of cooling and heating needed formaintaining the furnace interior wit-hin a :desired temperature range,the solid resistor furnace is limited in operation to the temperature atwhich the resistor melts or begins to experience serious chemical attackby the atmosphere of the furnace chamber, while a liquid resistorfurnace can be heated no further than the temperature at which theliquid resistor vaporizes. While fairly easy to re-start after a periodof operation, an electron Ebeam furnace nonetheless presents seriousdifficulties in temperature control and is operable only under arelatively very high vacu-um. Current flow to an arc furnace is easilystopped Iby opening a switch, but the narrow, ionized zone forming theconducting medium between the electrodes disappears im- -mediately uponcessation of the electrical flow, and the current cannot be restartedsimply by closing the switch. Instead, the electro-des must be movedinto Contact `with each other and then separated slightly to draw thearc; or electrical equipment must be supplied which will yield a specialstarting voltage high enough to provide an initial spark across theelectrode gap, Temperature control thus tends to be difficult andunwieldy in an arc furnace.

All the previously employed furnaces have ybeen Ibeset with thedisadvantage of large temperature gradients within the furnace chamberwhich seriously limit furnace efficiency. This problem is especiallycritical in electron beam furnaces and arc furnaces and is alleviated ina solid resistor furnace only Iby making the resistor area quite largein relation to the furnace interior and to the electrical power input.Temperature gradients between a liquid resistor and the material to beheated in the furnace are undesirably large except where the material tobe heated can be immersed in the liquid or is melted to itself form theliquid resistor. In the case of an arc furnace, all the heat isgenerated in the small region including the electrode tips and the arcbetween them, with most of the heat originating in the electrode tipsrather than in the arc. As in the case of a solid or a liquid resistorfurnace, heat distribution in an arc furnace must :be by radiation,convection, and conduction, and the efficiency of heat distribution fromthe small zone of heat origination in an arc furnace therefore isundesirably low. This undesirability is further aggravated where directcurrent is employed in an arc furnace, for such operation results instill further localization of the zone of origin of the heat in that theanode tip produces twice as much heat as the cathode.

Further difficulties arising in the operation of arc furnaces arerelated to the unavoidable occurrence of electrode deterioration. Whenunder D.C. operation of graphite electrodes, vaporized carbon passesfrom the cathode to the anode and is deposited on the tip of the latter.This carbon button interferes with arc propagation and all toofrequently falls away from the anode into the yfurnace charge, intowhich it enters as a contaminant. In addition, the current density atthe electrode tips becomes excessively high .under operation ywitheither direct or alternating current, `and the resulting high rate ofconsumption of the electrode material fills the furnace with vaporswhich contaminate the material heated in the furnace.

It will be evident that it is most desirable to provide a method ofoperating a furnace yielding advantages of previous furnace operatingmethods while obviating their disadvantages.

It is, accordingly, a major object of the present invention to providegreatly improved uniformity of temperature Within an electric furnace.

Another object is to provide improved ease and efliciency in temperaturecontrol of an electric furnace without reso-rt to a solid or liquidresistor element or the necessary utilization of a high vacuum.

A further Iobject is to provide for the attainment of highertemperatures than are possible in a solid or liquid resistor furnacewhile obtaining improved temperature distribution in the furnace chamberand efficient temperature control without the need `for moving thefurnace `electrodes or employment of special starting voltages.

Yet another object is to provide furnace operation wherein an inert,gaseous plasma fills the furnace chamfber and serves as the electricalresistance element.

A still further object is to reduce greatly the deterioration of theelectrodes in an electric 4furnace and the contamination of the furnacecontents by vaporized or deposited portions of the electrodes.

Still another object is to provide purging `and washing of a melt in afurnace chamber by an inert gas which serves as a plasma resistanceelement filling the furnace cham-ber.

Other objects and advantages will be apparent from the specificati-onand claims and Ifrom the accompanyingV drawing illustrative of theinvention.

In the drawing:

FIGURE l is a front elevation, in central longitudinal section, of afurnace suitable for practice of the present invention, the electrodesbeing `shown in position for preheating the furnace interior;

FIGURE 2 is a view similar to FIGURE 1 but only partially in section toyshow the gas-tight door of the mold compartment and yfurther showing amodification for effecting temperature control of the furnace, theelectrodes being positioned for plasma resistance operation of thefurnace;

FIGURE 3 is a vie-w similar to FIGURE 1 `and showing a secondmodification for effecting temperature control, a mold being shown inplace to receive the molten metal;

FIGURE 4 is `an exploded View of the electrode holder and `associatedparts;

FIGURE 5 is an oscilloscope trace of the voltage between electrodesduring A.C. arc operation; and

FIGURE 6 is an oscilloscope trace of the voltage between electrodesduring plasma resistance operation.

Briey described, the invention comprehends the method of heating anenclosure comprising the provision and maintenance in the enclosure ofan atmosphere of substantial (for example, at least 15%) argon content.This argon-enriched atmosphere is heated until at least some thermalionization occurs generally throughout the chamber, whereupon a pair ofelectrodes are provided in the chamber with a spacing greater than themaximum gap over which, at a given operating potential and in air (or ina cold, argon-enriched atmosphere) an arc could be propagated. Where theoperating potential is, for example, 40 volts, a convenient andeffective spacing of the electrodes is of the order of 6 inches,although a somewhat smaller spacing is not harmful and a larger spacing,where the furnace dimensions permit, is acceptable. The operatingpotential is applied across the electrode gap to obtain an electrical owthrough the ionized material distributed throughout the enclosure, thusutilizing the enriched atmosphere as a resistance element. Temperaturecontrol is obtained as more fully described in later paragraphs. Theinvention further comprises means for carrying out the above method.

With reference to FIGURE l, the electric furnace comprises a crucible 10preferably made of a dielectric material or provided with a dielectriclining. The crucible is of porous construction in order to permit thepassage of a gas under pressure from its exterior surface, in particularfrom its bottom, to its interior cavity or chamber 11.

The crucible 10 is contained in a housing 12 with walls and partitionsof metal or other heat-resistant material which enclose all the crucibleexterior surface in an airtight manner. Spaced slightly above thehousing lower wall 13 is a transverse partition 14 upon which the bottomsurface of the crucible 10 rests. An opening 15 somewhat smaller thanthe diameter vof the lower surface of the crucible 10 is formed in thepartition 14 and is overlapped around all its periphery by the poroussurface of the crucible 10, which thus has communication with a plenumchamber 16 enclosed within the housing 12 between the housing lower wall13 and partition 14. The housing upper side or fwall 17 is spacedslightly above the crucible 10 to form therebetween a space which, likethe space between the sides of the crucible 10` and the housing sidewalls 18, 19, is filled with an insulating material 20, preferably aceramic, which seals off the outer surface of the crucible at its topand sides. Since lthe lower surface of the crucible 10 is in turn closedoff by the housing lower wall 13, the housing 12, including the ceramicinsulating material 20', sealingly isolates all the exterior surface ofthe cr-ucible from the atmosphere. The plenum chamber 16 is filled witha porous or loose insulating material such as spherical, hollow grainsof fused alumina 21 followed by an -outer layer of rock wool 72. Theplenum chamber 16 is connectible with a source of an inert gas7specifically argon, through a tube 22 and thus, in cooperation with theporous crucible 10, is a means for maintaining an increasedconcentration of argon in the crucible. An opening `66 through thehousing upper side 17 and adjoining insulating material 20 communicateswith the crucible cavity 11 and permits withdrawal of melted materialsfrom the crucible 10.

The supporting frame 23 includes a pair of vertically extending, fixedmembers 24, 25 Iwhose upper ends are spaced to either side of thehousing 12. A pair of f1ttings 26, 27 are rigidly mounted on the housing12, one at each side of the cr-ucible 101, and each fitting 26 or 27pivotally engages a respective supporting member '24 or 25 A passage 28extends axially through the fitting 26 and through the housing wall 18,insulation 20, and crucible 10 into the crucible interior to permit themou-nting and variable extension into the crucible chamber 11 of anelectrode 30. A second electrode 31 is similarly extensible into thechamber through -a similar, second passage 29 at the other fitting 27.The electrodes 30, 31 preferably are of such length that, with theirinner ends in contact with each other, their outer ends extendexteriorly of the fittings 26, 27.

FIGURE 4 shows a representative one of the fittings 26, 27, whichincludes a bearing ring 32, insulating gasket 33, electrode holder 34,and end cap 35. The bearing ring 32 is of tubular construction withspaced, circular end tianges 37, 38. The innermost end iiange 37 isrigidly mounted, as shown in FIGURE l, on the housing, while the otherange 38 is drilled for attachment of the circular electrode holder 34.The latter has a central, tubular portion extending away from thebearing ring 32 and is provided 'with a lug 39 or equivalent forattachment tof the electrical lead 40 through which electrical power issupplied to the associated electrode 30.

The electrode 30 has a snug, sliding fit in the electrode holder 34. Thepassage 28, where it extends through the bearing ring 32, is of largerdiameter than the electrode 30' and the same is true of the portion ofthe passage 28 extending, as may be seen in FIGURE l, from the bearingring 32 into the crucible chamber 11. The electrode, therefore, isspaced from the wall of the passage 28 except at the holder 34, by closesliding contact with which it is afforded support and electricalconnection with the lead 40. The spacing of the electrode 30 from thewall of the passage 28 is sufiicient to prevent arcing to the bearingring 32 at operating voltages.

Means electrically isolating the electrode holder 34 of the one fitting26 or 27 from that of the other include the insulating gasket 33 placedbetween the bearing ring 32 and electrode holder 34 and insulatingbushings 42 between the bearing ring tiange 32 and the electrode holderstuds 41 which extend through the bearing ring flange 38. Insulatingwashers 43 are placed between the bearing ring flange 38 land nuts 44which are run down on the studs 41 to clamp the electrode holder 34 inairtight manner on the bearing ring 32.

A plurality of cap-mounting studs 45 extend voutwardly from the face ofthe electrode holder 34 and engage corresponding openings in a flange ofthe end cap 35. When wing nuts 46 are tightened down on the studs 45,the end cap 35 is pulled into close, airtight engagement 'with theelectrode holder 34 and encloses the protruding outer end (see FIGURE 1)of the electrode 30. The passages 28, 29, thus may be sealed off, asshown in FIGURE 2, from the atmosphere to permit furnace operation undera partial vacuum. The caps 35, 36 must be long enough to house theelectrode outer ends when the electrodes 30, 31 are fully separated, aswill be described, for plasma resistance operation. f

Each supporting member 24 tor 25 terminates at its upper en-d, as shownin FIGURE 4, in a lower trunnion half 47 which lies between the flanges37, 38 of and receives the tubular portion of the bearing ring 32. Thetrunnion upper half `48 is bolted to the trunnion lower half 47 tocomplete the pivotal mounting of the housing 12 (FIGURE l) ton the twosupporting members 24, 25.

The leads 40, 49 are shown for representation of a source of electricalpower at a given, desired operating voltage connectible, as described,to the electrodes 301, 31 for supplying the operating potential acrossthe gap between the electrodes. The operating voltage preferably isrelatively low, for example 40 t-ol 70` volts, and the amperageaccordingly is high. 'I'he switch 50 is provided in `the lead 40 formaking and breaking the electrical connection between the electrodes 30,31 and the leads 40, 49 extending to the power source.

'Ilhe modifications shown in FIGURES 2 and 3 include means, such as aradiation or optical pyrometer 51, responsive either -to total radiationor to a particular portion of the spectrum of the energy emitted in thehot interior of .the crucible 10. In FIGURE 3, the temperature sensingmeans 51 is connected as by a linkage represented at 52 to the switchingmeans 50 `and is responsive for opening the switch 50 when the furnaceinterior reaches -a desired maximum temperature above 2800 F. yand forclosing the switoh 50` when the furnace has cooled to a desired lowerlimit above 2800 F. In both FIGURES 2 and 3, the pyrometer 51 receivescrucible radiations through a vacuum-tight sight hole 53 extending fromthe Crucible cavity 11 to the exterior of the housing 12. A sight glassholder 54 (FIGURE 2) is attached in gastight manner in the outer end ofthe sight hole 53 and a sight glass is sealingly attached on its outerend. A gas, for example argon, is owed into the sight glass holder 54near its outer end through an inlet tube 55 to keep furnace vapors sweptout of the sight hole 53, thereby preventing clouding of the sightglass.

The modification shown in FIGURE 2 employs a branched tube 22A leadingfrom the plenum chamber 16. One branch 56 leads, as in the othermodifications, to the argon supply through a valve 57. The other branch58 leads through a valve 59 to a supply of a gas other than argon (forexample, nitrogen) which may be used to dilute lthe argon supply in theCrucible chamber 11. The pyrometer 51 is connected as at 60 to the argonflow regulating valve 57 to control the argon ow as will be described.Alternatively, the pyrometer 51 may be linked, as will become evident,with `the valve 59 controlling the flow of the other gas. Each of thevalves 57, 59 thus constitutes -a means for varying the concentration ofthe argon in the Crucible chamber 11.

To receive the Crucible contents 67, as described later, a mold isplaced on the housing upper side 17 as shown in FIGURE 3. Means areprovided for reducing atmospheric pressure, during yany stage of furnaceoperation, in the Crucible Chamber 11 as well las about the mold 61 andat the housing upper side 17. This means includes a compartment 62containing .the mold 61 and formed by a tcp wall 63 which is associatedwith the housing top side 17 and extensions of the housing side wallsincluding the walls 18, 19. A tube 64 opening into this Compartment 62leads to a vacuum pump or equivalent (not shown), and the compartment isclosed off, when desired, by a gas-tight door 65 (FIGURE 2).

In operation of the furnace for heating to `a high ternperature theenclosure 11 formed by the Crucible 10, an argon-enriched atmosphere isprovided and maintained in the Crucible. Utilizing the preferred meansfor accomplishing this step, the valve 57 (FIGURE 1) is opened asrequired for directing a desired flow of argon through the tube 22 intothe plenum chamber 16, from whence it passes through the porous materialof the Crucible into the Crucible interior 11. A pure 'atmosphere ofargon is not essential to plasma resistance operation, andthe argoncontent can be varied widely as long as the minimum concentrationrequired for .filling the furnace with a plasma resistor is maintained,a preferred minimum concentration being of the order of argon by weight.While, for reasons which will become apparent, it is preferred tointroduce the argon as described, other modes of introducing it areacceptable. As an example, suicient argon may be flowed into the furnacethrough the sight hole purging tube 55 (FIGURE 2), through another boreequivalent to the sight hole 53, or through a conduit communicating withone or both the passageways 28, 29 (FIGURE 1) through which the pair ofelectrodes 30, 31 variably extend into the Crucible 10. The setting ofthe valve 57 is adjusted to obtain and maintain the desiredconcentration of argon during the remaining operation of the furnace.

A gas, in its normal state, is a very poor Conductor of electricity andbecomes a conductor only when it Contains `enough free electrons andions to serve as carriers for a current. The zone including an ordinaryarc between electrodes contains ionized materials which are ofsignificantly high carbon content where graphite electrodes are used andwhich are kept heated to ionizing temperature in part by resistanceheating in the arc but chiey by the heat emitted by the electrode tips.As the electrodes are more widely separated, a gap is reached which isthe maximum gap over which, at a given potential, an arc can bemaintained in, for example, air, carbon dioxide, nitrogen, etc. Whenthis gap is exceeded, the arc breaks, for the ionization of the mediumbetween the electrodes becomes insuflicient to Imaintain current llowbetween them. The maximum gap is relatively small, `and an arc cannotreadily be maintained over a gap exceeding about `one-half inch at 40volts or one inch at 70 volts, the gap being correspondingly smaller orgreater as the voltage is decreased or increased. All the heating thusis localized to the small zone including the extreme tips of theelectrodes and the narrow `arc between them.

To provide more and other than ordinary arc heating, therefore, theargon-enriched atmosphere in the enclosure 11 provided by the Crucible10 is heated until some of it, generally throughout the enclosure, isthermally ionized. In this manner, the resistance between any two pointsspaced apart within the Crucible `atmosphere is reduced, a reduction tothe order of 0.01 ohm per inch of spacing being suliicient inrepresentative applications. Satisfactory ionization is obtained bybringing the cru- Cible atmosphere up to or above approximately 280()aF., and this is done in any convenient way resulting in the desiredCrucible interior temperature. The heating of this atmosphere iseffected, for example, by striking lan arc between the electrodes 3l),31 with a given, alternating or direct current potential across them andwith the electrodes spaced at least slightly less than the maximumspacing at which the arc can be maintained between them, at the givenpotential, in air or in the cold, argon-enriched atmosphere. The arc ismaintained until a temperature of the Crucible wall and interior isreached at which thermal ionization occurs in the argon of the Crucibleatmosphere outside the arc. As measured by an optical pyrometer, thistemperature is very near 2800 F.; a temperature of 2775 F., for exampleis not sufficient.

The argon-enriched atmosphere having been sufi'iciently heated, it isnecessary to provide a pair of electrodes in the enclosure. This willalready have been accomplished where, as described, the electrodes 30,31 are themselves utilized to provide preliminary heating by arcoperation. Where the Crucible preliminary heating is brought `about inanother manner, the electrodes 30, 31 nonetheless must be provided, inthis Case in addition to the preliminary heating means. Electrodes 30,31 of graphite or of tungsten have yielded excellent results. Plasmaresistance operation is begun by increasing the spacing of theelectrodes 30, 31 to provide a gap greater than the maximum gap overwhich an arc is propagable in air at the given potential. Ordinarily,the electrode spacing is several times, i.e., at least twice, thismaximum spacing, and actually a relatively wide spacing is desired toencourage Current ow generally throughout the chamber 11 through thethermally ionized atmosphere filling the same. For example, at anoperating potential of 40 volts, a 6-inch spacing between electrodes 30,31 is entirely feasible and is conducive of good results. A larger gapmay readily be employed, for example a gap of 5 inches or more for each25 volts of operating potential, and maximum gap size is ordinarilylimited, under lowvoltage, high-amperage operation, only by thedimensions of the Crucible chamber 11.

Since the ionized, gaseous resistance element or plasma substantiallyfills the entire chamber 11, separating the electrodes 30, 31 asdescribed and placing the given, selected voltage across them resultsnot in an ordinary arc but in a much more diffuse ow which occursthroughout the argon-enriched Iatmosphere tin the enclosure v11. Sincethe ow is dilfuse, it will be found that a potential (at least a largefraction of the operating voltage) is placed on a conductive probeplaced anywhere in the Crucible cavity 11 or even in the outflow ofplasma, like a tongue of flame, which ordinarily extends a shortdistance outside the Crucible opening 66. The entire atmosphere in thelCrucible cavity 11 therefore is conductive. Before reaching atemperature at which plasma-resistance operation is possible, currentflow out of the electrodes 30, 31 is only at the electrode tips; currentdensity at the tips therefore is high, and the tips become very hot anddecompose with undesirable readiness. When all the atmosphere becomesconductive, current flow is out of all the electrode surfaces (notmerely the tip surfaces) within the furnace chamber 111, and currentdensity at the tips of the electrodes 30, 31 is radically lessened,although current liow remains fully as great as during arc operation atthe same voltage. Although the furnace temperature increases underplasma operation, electrode tip temperature is markedly decreased andvaporization of the electrodes 30, 31 is radically reduced Where notvirtually eliminated. Besides the advantages of greatly extendedelectrode life, carbon vapor contamination or adulteration of thematerial 67 heated in the furnace is obviated, and even under D.C.operation there is no Carbon button built up on the anode and likely tofall into the melt. Most of the heat is generated by passage of theelectrical current through the plasma rather than origin-ating at theelectrodes 30, 31; hence, this heat is ygenerated throughout the chamber11 rather than at and immediately between the electrode tips. As aconsequence, temperature lis comparatively yvery uniform throughout thefurnace chamber 11 and there is little if any reliance on conduction,radiation, and convection for reducing temperature gradients within theCrucible 10. It is of interest that the current flow through the plasmaresistor differs in kind from that through an arc. As shown in FIGURE 5,the flow through electrodes separated by a gap spanned by `an arc, anA.C. flow being shown by way of example, results in or is accompanied bysharp voltage fluctuations (voltage across the electrodes being shown bythe line 70) which greatly vary the wave-form of the power supplyvoltage, in this case a sine wave. FIGURE 6 shows the voltage acrosselectrodes to which an A.C. current is fed during plasma resistanceoperation. Without entering into discussion of the causes of the rapidand violent variations of voltage during arc peration, it will be notedthat the voltage during plasma resistance operation follows the puresine wave of the power source. The plasma behaves, therefore, purely asa resistor of constant Value. The arc fiow is of another kind andnature, as evidenced by the voltage fluctuations.

The plasma resistor, resulting from ionization within an atmosphereenriched with one of the noble, inert gases, offers none of the problemspresented by other resistors. The plasma is entirely compatible with allmaterials which may be employed in constructing the furnace or he-atedyin the same; it does not chemically attack other materials, nor is ititself oxidized or otherwise affected by air. In fluid state, .fit isnot adversely affected by further increase in temperature above 2800 F.and hence is not subject to a melting or vaporization such as thatfwhich, in furnaces employing a solid or liquid resistor, limits theupper range of operating temperature. There are no resistor maintenanceproblems, it being necessary only to ensure an adequate concentration ofthe plasmaproducing gas in the crucible. Operating pressures are in noWay critical, as in an electron beam furnace: the plasma resistor hasbeen operated efficiently from pressures above atmospheric down topressures as low as 0.03 mm. of mercury.

Furthermore, it is important to note that the argon serves not only toprovide the plasma for heating, it also protects the furnace interiorand its contents from reaction with atmospheric gases 'and furthermoreserves the important and valuable function, entering the chamber as itdoes through the porous crucible material at the bottom of the .cruciblechamber, of bubbling up through the heated material 67 and scrubbing thesame of impurities when it is melted as in FIGURE 3.

Whereas `an arc furnace is limited to the maximum temperature attainableat the heated material by heat brought to the latter, from` the arczone, by conduction, convection, and radiation, no such limitationexists when heating with the plasma resistor, for heating occursthroughout the furnace cavity. The furnace operating temperaturetherefore is limited only by the ability of the Crucible material towithstand melting. Meanwhile, it will be seen that temperature controlwhen using the plasma resistance element is more effective and more:easily attained than in arc and electron beam furnaces and iscomparable with temperature control in solid and liquid resistorfurnaces.

According to a preferred feature of the method of operating an electricfurnace whereby the furnace interior l1 vis maintained within a desiredtemperature range whose upper and lower limits lie above 2SGO F., thecurrent flow through the electrodes 30, 3l and plasma is interruptedwhen the temperature of the furnace interior reaches the desired upperlimit. This is readily accomplished by opening the switch 50.

The furnace-is then allowed to .cool for a period of time sufhcient forits temperature to be reduced to the desired lower limit. At this time,while maintaining the above-described electrode spacing for plasmaresistor operation, the given operating voltage is again applied acrossthe electrodes 30, 31 to resume the current flow. Closure of the switch50 is sufficient to effect this flow, for while the furnace interior liremains above 2800o F., the argon-enriched atmosphere within the furnaceremains .conductive and current flow commences immediately upon placingthe operating voltage across the electrodes. There is no need, as inordinary a-rc furnace operation, for touching the electrodes 3ft, 31together or for added equipment for providing a high-voltage startingspark between spaced electrodes. The sole requisite for operation, asabove described, for controlling temperature within the electric furnacetis that the current fiow be resumed before the argon-enrichedatmosphere in the furnace interior has cooled below 2800 F., for uponpassing below this temperature the crucible atmosphere is no longersufficiently ionized to re-star-t the flow without resort to touchingthe electrodes 30, 31 together or reducing their spacing to an ordinaryarc gap and employing a high, special starting voltage.

Temperature within the electric furnace also is effectively controlledby increasing the argon content of the Crucible atmosphere to raise thecrucible temperature and decreasing the argon content to lower thetemperature. Where the crucible l0 is operating, for example, with thecruci-ble opening 66 exposed to the atmosphere, relatively Cool andhence heavy air outside the crucible tends to flow down through theopening 66 and thus to dilute the argon-enriched atmosphere in thecrucible. A given, lconstant flow of argon into the Crucible through thetube 22 or 22A therefore will tend to result in a-n equilibrium beingreached at which the argon content in the crucible l0 is relativelyConstant. Adjusting the valve S7 to provide a greater argon flow furtherenriches the crucible atmosphere and concurrently raises itsconductivity. The resulting increase in amperage of the current passingthrough the crucible atmosphere, voltage being held constant, increasesthe rate of evolution of heat. Dilution of the crucible atmosphere withair or other gases increases its resistance and diminishes the currentflow, thereby lowering the furnace temperature. The argon content mustnot be lowered so greatly as to lose the plasma resistance mode ofoperation, and it has been found that this operation is still wellretained when the argon content has dropped to 15%. By the same token,the temperature reduction cannot proceed below 280()o F., for the plasmaresistance operation will be lost below this temperature.

Automatic temperature control is obtained by operation of themodification shown in FIGURE 3. The temperature sensing means, forexample the optical pyrometer 5l, senses the temperature within thecrucible 10 -through the `sight hole 53 and responds to occurrence ofthe maximum desired temperature `by opening the switch 50 by actuatingthe linkage 52. In response to the furnace temperature having fallen tothe `desired lower limit, the temperature sensing means actuates thelinkage to close the switch and thus to re-start the current flow.

In the modification shown in FIGURE 2, the temperature sensing meansresponds to temperatures within the crucible chamber to open and closethe argon flow control valve 57 through the linkage 60, the valve beingmore widely opened to increase argon ow when the furnace temperaturefalls below a -desired value and closed to decrease argon flow when thefurnace temperature becomes excessive. Alternatively, the linkage 60 canbe connected lto the valve 59 in the branch 58, whereupon the argon flowis set manually to a constant value and the argon concentration in thecrucible varied by action of the sensing means 51 on the second valveS9. The second valve S9 is opened to admit a gas other than argon (forexample, nitrogen) through the branch 58 and thus to dilute the argonflow into the crucible chamber 11. The dilution results in a lessened`current flow and lowered temperature. The dilution must not beexcessive, for too much nitrogen, air, carbon dioxide, etc. will soreduce the argon concentration and increase the resistance of thecrucible atmosphere as to result in abrupt loss of plasma resistanceoperation. Closing the valve 59 permits the argon concentration to riseand thus raises the temperature in the crucible 10.

Whenever operation of the furnace under a partial vacuum is desired, themold compartment 62 is rendered airtight by closing the door 65 (FIGURE2). The caps 35, 36 are sealingly mounted on the electrode holders atthe fittings 26, 27 to prevent the inflow of air around the electrodes30, 31 and the compartment 62 and crucible chamber ill are evacuated, tothe extent desired, by connecting the tube 64 to a 'vacuum pump orequivalent. According to one frequently employed sequence of operations,the metal 67 (FIGURE 1) or other material to be heated is introducedinto the furnace and melted at atmospheric pressure with, of course,argon enrichment. 'Ihe mold `61 (FIGURE 3) then is placed on the housingupper side 17 and in register with the crucible pouring hole `66. Asupport plate 68 is placed on top of the mold `61, and the mold isclamped in place by fastening means such as a pair of air-drivenactuators 69 Whose piston rods are extensible, upon air being suppliedto the actuators, into engagement with the support plate. The door 65(FIGURE 2) is put in place to render the mold compartment 62 (FIGURE 3)airtight, and the compartment 62 and crucible chamber 11 are evacuatedthrough tube 64. After further heating of the melt 67, where desired,the housing 12 is rotated 180 degrees on the supporting members 24, 2S,thereby pouring the liquid metal 67 (FIGURE 3) into the cavity of themold 61. After the metal has solidified, the furnace is rotated back toits original position. After introducing argon to return the cruciblechamber 11 to atmospheric pressure, the gas-tight door 65 is opened andthe mold is removed from the compartment.

While only certain particular embodiments and modifications of theinvention have been described herein and shown in the accompanyingdrawing, it will be evident that fur-ther modifications are possiblewithout departing from the scope of the invention.

We claim:

1. The method of heating an enclosure comprising:

providing and maintaining in the enclosure an atmosphere of at least 15%argon content;

heating substantially all the atmosphere in the enclosure to atemperature at which some of the argon of said atmosphere is thermallyionized throughout the enclosure;

providing a pair of electrodes in the enclosure and spacing them apartto provide between them a gap greater than the maximum gap over which anarc is propagable between the electrodes in air at a given potential;and applying said given potential across the gap to obtain a flow ofelectrical current through the atmosphere in the enclosure.

2. The method of heating an enclosure comprising:

providing and maintaining an argon-enriched atmosphere in the enclosure;heating the atmosphere in the enclosure to a temperature at which theelectrical resistance of the atmosphere is lowered to the order of 0.01ohm per inch between spaced points substantially anywhere in theatmosphere by ionization of argon of the atmosphere;

providing a pair of electrodes in the enclosure spaced to provide a gapbetween them greater than the maximum gap capable of supporting an arcbetween the electrodes in air at a given potential;

and applying said given potential across the electrode gap `to obtain adiffuse oW of electrical current through the argon-enriched atmospherein the enclosure.

3. The method o-f operating an electric furnace having a crucible and apair of electrodes variably extensible into the crucible, said methodcomprising:

providing and maintaining in the crucible an atmosphere of at least 15%argon content;

placing a given potential across the electrodes and providing an arcbetween them by spacing them at a given spacing less than the maximumspacing at which, at the given potential, an arc can be maintainedbetween them before the atmosphere in the crucible is heated;

heating the crucible by maintaining said arc until the crucible interiorreaches a temperature at which ionization occurs in the argon of saidatmosphere well outside the arc;

and increasing the spacing between the electrodes beyond said maximumspacing.

4. The method of operating an electric furnace having a dielectriccrucible, said method comprising:

providing a pair of electrodes made of a material selected from thegroup including carbon and tungsten and Variably extensible into thecrucible; maintaining in the crucible an atmosphere of at least 15 argoncontent;

providing an arc between the electrodes by placing a given, alternatingpotential across the electrodes and spacing them at a given spacing nogreater than the maximum spacing at which an arc can be maintainedbetween them, at the given potential, before the atmosphere in thecrucible is heated;

heating the crucible and its contents by maintaining v said arc until atemperature is attained at which ionization occurs well outside the arcin the argon of the atmosphere in the crucible;

and increasing the spacing between the electrodes to a spacing whichexceeds twice said maximum spacing. l5. The method of operating anelectric furnace comprising:

providing and maintaining in the furnace an argonenriched atmosphere;heating the furnace interior and substantially all the argon-enrichedatmosphere to a temperature in excess of 2800 F.;

providing a pair of electrodes in the furnace spaced by a gap of morethan 5 inches;

and passing a ow of electrical current through the argon-enrichedatmosphere by applying a power source with a potential of less than 200volts across the gap between the electrodes.

6. The method of maintaining the interior of an electric furnace withina desired temperature range having an upper and a lower limit both above2800o F., said method comprising:

maintaining an argon-enriched atmosphere in the furnace;

providing in the furnace a pair of electrodes;

heating the furnace interior in excess of 2800 F.;

passing a flow of electrical current through the yargonl 1 enrichedatmosphere by applying a given potential across the electrodes with theelectrodes spaced a given spacing which is a plurality of times thespacing across which an arc can be maintained at the given voltage inair;

interrupting the current flow through the argon-enriched atmosphere whenthe furnace interior reaches the upper limit of the temperature range;

and leaving the electrodes spaced at said given spacing and applyingsaid given voltage across them to resume the flow of electrical currentthrough the argon-enriched atmosphere before the furnace interior hascooled to the lower limit of the temperature range.

7. The method of controlling the temperature within an electric furnacehaving variably spaceable electrodes cornprising:

heating the furnace interior above 2800 F. by striking an arc betweenthe electrodes;

maintaining an argon-enriched atmosphere in the furnace;

spacing the electrodes to provide between them a gap which is aplurality o-f times the maximum gap at which an arc is sustainablebetween the electrodes in air at a given potential;

passing a flow of electrical current through the argonenrichedatmosphere by imposing said given potential across the electrodes;

and increasing the argon content in the furnace atmosphere to raise thetemperature within the furnace and decreasing the argon content in thefurnace atmosenriched atmosphere at a given potential with theelectrodes separated by a gap of at least one inch for each 25 volts ofsaid potential.

9. The method set forth in claim 8 and further cornprising:

cutting off the electrical current flow through the argonenrichedatmosphere when the furnace temperature reaches a desired upper limitabove 2800 F.; and

maintaining said electrode gap and resuming the current flow through theargon-enriched atmosphere by applying said given potential across thegap when the furnace temperature has fallen to a desired lower limitabove 2800 F.

References Cited in the le of this patent UNITED STATES PATENTS1,026,269 Knaufr" May 14, 1912 1,137,295 Stearns Apr. 27, 1915 1,310,079Hechenbleikner July 15, 1919 1,499,922 Hadaway July 1, 1924 1,572,666Marsden Feb. 9, 1926 1,727,986 iones Sept. 10, 1929 1,749,396 SchylanderMar. 4, 1930 1,961,496 Holmes June 5, 1934 1,985,492 Frohmuth et al Dec.25, 1934 2,147,070 Weinheimer et al Feb. 14, 1939 -2,179,818 Hampton etal. Nov. 14, 1939 2,285,837 Ridgway June 9, 1942 2,454,576 Slack Nov.23, 1948 2,667,561 Sehoenwald Jan. 1, 1954 2,726,278 Southern Dec. 6,1955 2,782,245 Preston Feb. 19, 1957 3,004,137 Karlovitz Oct. 10, 19613,023,295 Johnson Feb. 27, 1962 3,048,687 Knowles Aug. 7, 1962 FOREIGNPATENTS 152,176 Great Britain Oct. 14, 1920 633,020 Great Britain Dec.5, 1949

1. THE METHOD OF HEATING AN ENCLOSURE COMPRISING: PROVIDING ANDMAINTAINING IN THE ENCLOSURE AN ATMOSPHERE OF AT LEAST 15% ARGONCONTENT; HEATING SUBSTANTIALLY ALL THE ATMOSPHERE IN THE ENCLOSURE TO ATEMPERATURE AT WHICH SOME OF THE ARGON OF SAID ATMOSPHERE IS THERMALLYIONIZED THROUGHOUT THE ENCLOSURE; PROVIDING A PAIR OF ELECTRODES LIN THEENCLOSURE AND SPACING THEM APART TO PROVIDE BETWEEN THEM A GAP GREATERTHAN THE MAXIMUM GAP OVER WHICH AN ARC IS PROPAGABLE BETWEEN THEELECTRODES IN AIR AT A GIVEN POTENTIAL; AND APPLYING SAID GIVENPOTENTIAL ACROSS THE GAP TO OBTAIN A FLOW OF ELECTRICAL CURRENT THROUGHTHE ATMOSPHERE ILN THE ENCLOSURE.